It is already known to press a sliding or rolling tool tip, which is configured, for example, as a ball, a hemisphere, a disc, or a wheel, onto a workpiece and to move over its surface with corresponding rotation of the workpiece. A procedure of this type is usually referred to as roller burnishing, burnishing, or deep rolling, and results in chipless smoothing of the surface roughness of the workpiece and the generating of compressive residual stresses in the workpiece. The compressive residual stresses can be influenced in their strength by the pressing force of the tool tip against the workpiece, and in their depth profile by the size of the contact surface, e.g., the radius of the tool-tip ball or hemisphere. The hardness and the wear resistance of the surface, its dynamic stability (fatigue limit, vibration resistance) and its percentage of load bearing area can be increased; crack formation is made more difficult by compressive residual stresses. Smooth cold-worked surfaces also have an increased corrosion resistance.
With known tools the hard tool tips are pressed onto the workpiece by disc spring packages or by a water- or oil-hydraulic system. In spring-loaded versions the tool tip is configured as a ball made from ceramic, rolling over the surface, or as a hemisphere made from diamond, sliding over the surface; in hydraulic solutions preferably as a hydrostatically supported and rolling ceramic ball.
In the case of disc spring packages a particularly compact design and a rapid force buildup is achieved in the workpiece-tool contact. However, even with small deviations from the target pressing-force setting, which can arise from positioning errors, such as from the workpiece contour, a significant deviation in the machining force is to be observed. The hysteresis of the disc spring packages during loading and releasing likewise leads to fluctuating machining forces. The very hard spring-loading tends, with corresponding workpiece roughness, to a hard vibrating of the tool tip, which influences the surface quality, but in particular decreases the service life of the tool tip.
In hydraulic systems, a stroke range with constant pressing force is realized by using a cylinder integrated in the tool. This system indeed has an adjustable and constant pressing force, but has other disadvantages. For example, a hydraulic unit is needed and the way the hydraulic unit is connected to the tool usually precludes the use of an automatic tool changer in the machine tool. Furthermore, many machine tools work with a water-based emulsion as cooling lubricant, and thus using oil in the tool may be undesirable due to the risk of it mixing into the emulsion. In such cases the hydraulic system must be driven with emulsion, which is generally undesirable. For example, emulsions have unfavorable lubrication and pressure resistance, and may come to contain hard or abrasive particles that, despite filtration, lead to a service-life reduction of the hydraulic unit as well as the tool. Sliding the tool tip over the particles or rolling over the particles with the tool tip can damage the tool tip and/or the workpiece.